Various types of equipment receive information in analog form. Such equipment includes process control equipment, measuring instruments, communication equipment and a wide variety of other equipment. Digital computers and data processing systems often receive input parameters in analog form from such equipment and convert these parameters to digital form for processing in the computer or the data processing equipment. After the analog information has been converted to digital information and has been processed, the output information from the digital computer or the data processing equipment is often converted to analog form. By converting the digital information to analog form, the user can assimilate the information in ways which would be difficult if the information remained in digital form.
For example, digital information at a first telephone line interface can be converted to analog form at that interface. The processed information can be transmitted in analog form through telephone lines and received at a second interface distant from the first. The information received at the distant interface can then be converted to digital form. The digital information can comprise computer data or digitized video images.
A good example of the conversions discussed in the previous paragraph is in the recording and reproduction of music. The music is produced in analog form. It is converted to digital form by recently developed data processing techniques and is recorded on a medium such as a digital or a compact disc. When the music is to be reproduced, it is converted again to analog form because this is the form which operates sound transducers to give meaning to the listener when he hears the music.
As digital computers and data processing equipment have become widespread throughout industry and the office and have even become common in the home, the need for inexpensive, simple and reliable apparatus for converting information between digital and analog forms has become of increasing concern. A considerable effort has been devoted over a period of several decades to provide for converting apparatus which is simple, inexpensive and reliable. In spite of such efforts, the converting apparatus now in use does not meet such criteria.
The converting apparatus now in use also has other problems of long standing. For example, the converting apparatus now in use may not be monotonic unless it is quite expensive and complex. By "monotonic" is meant that digital information of progressively increasing value is converted to analog information of progressively increasing value without any decrease in the analog values as the digital value progressively increases. The converting apparatus now in use also has relatively high differential and integral nonlinearities unless the apparatus is quite expensive and complex. Integral non-linearities result from errors produced in a conversion between digital and analog values. Differential non-linearities result from errors produced in a conversion between digital and analog values over a relatively narrow range of such values.
The converting apparatus now in general use also has a problem of major proportions. This results when particular digital values are increased incrementally by a single digit. For example, problems in the converters now in use result when a binary representation of "511" is converted to a binary representation of "512". The results from the fact that the binary representation of "511" is represented by a value of 0111111111 and a binary representation of "512" is represented by 1000000000 where the least significant digit is at the right. As will be seen, the value of each binary digit changes when the decimal value changes from "511" to "512". As the binary values change from 0111111111 to 1000000000, discontinuities may occur because there is change between a binary value of "0" and a binary value of "1" at each binary position. These discontinuities may prevent the converter from being truly monotonic. This problem even exists in converters which are made quite complex in an attempt to overcome the problem.
There are other problems of long standing. For example, the number of binary bits of individual binary significance to be converted to analog values has been progressively increased in an attempt to enhance the resolution of the information being transmitted and received. As the number of binary bits has been increased, the complexities in the construction of the digital-to-analog converter has more than correspondingly increased. Furthermore, the rate of conversion has tended to decrease, partly as a result of such construction complexities.
It has been recognized that some of the problems discussed in the previous paragraph could be significantly resolved by providing the digital-to-analog converter with a recursive construction. In a converter with a recursive construction, a relatively simple construction block is provided on a generic basis and is repeated to convert successive bits of progressive binary significance.
In U.S. Pat. No. 4,857,929 issued to Henry S. Katzenstein on Aug. 15, 1989, for "Apparatus for Converting Data Between Analog and Digital Values" and assigned of record to the assignee of record of this application, apparatus is disclosed and claimed for converting between digital and analog values on a monotonic basis. Such apparatus is simple and inexpensive and reliable. It is able to operate with a high precision at high levels of power. As a result, the converting apparatus is capable of being used in areas not available for the converters of the prior art.
The apparatus disclosed and claimed in U.S. Pat. No. 4,857,929 also has other advantages of some importance. For example, it provides analog values of progressively increasing value as the digital value to be converted increases. It provides low differential and integral non-linearities within a single array of switches. It is also free of the effects of aging and changes in temperature. It also provides accurate and reliable conversions between digital and analog values at high frequencies with minimal error. It is relatively simple in construction because it is recursive. In other words, successive stages of the converter are formed from progressive ones of a single relatively simple building block. This converter is also advantageous because it provides an interpolation line. The interpolation line may be connected to the array to receive a current from the output member to be connected next to the particular line for increasing binary values. The interpolation line and an additional array means receiving binary values of reduced digital significance may be connected to provide an output current from the additional array means in accordance with the value of the binary bits of reduced binary significance and for introducing such current to the converter output in representation of the analog value of the binary bits of reduced binary significance. By connecting such array of recursive switches to the interpolation line, the complexity of the converter can be considerably reduced. This tends to lower the total parasitic switch capacitance and enhance the frequency of operation of the converter.
The primary problem limiting resolution in the converter of U.S. Pat. No. 4,857,929 has been the absence of a method for uniformly distributing the interpolation current from the first array to the plurality of input nodes of a second array. Simple direct connection of a first array to a second similar array has produced results with differential linearity several bits less than the anticipated sum of the binary bits driving both arrays.
One object of the present invention is to provide a converter with differential linearity that does not appreciably degrade as additional arrays are assembled to form the converter.
In one embodiment of the invention disclosed and claimed in U.S. Pat. No. 4,857,929, apparatus is provided for converting between binary and analog values. The apparatus includes means for providing a plurality of signals representing the binary values. Pluralities of control switches are also provided, the number of control switches in each plurality being directly related to the binary significance of the control switches in such plurality. The control switches may be paired to provide for a conductivity of one switch in each pair. Each binary signal and its complement are introduced to a corresponding plurality of control switches. The number of switch pluralities required in the array is then equal to the number of control signal binary bits.
The switches in U.S. Pat. No. 4,857,929 are connected in a repetitive or recursive array to provide paths through the conductive ones of the control switches. The switches are connected in a circuit with output members and a particular line. Since the output members are conductive, the control switches operate to introduce the current through the output members to the particular line. This provides for progressive increases in the number of the output members connected to the particular line with progressive increases in the binary value and for a connection to the particular line of the output members previously connected to the particular line with progressive increases in the binary value. The cumulative current through the output members connected to the output line is indicative of the analog value. Because the number of switches connected between the output line and the particular line increases with the number of pluralities of switches provided, the frequency response of the array degrades in proportion to the number of control binary bits.
It is another objective of this invention to significantly improve frequency response by providing pluralities, or rows, of switches that are controlled by several binary bits, thereby greatly diminishing the number of switch pluralities required in a new converter with equivalent resolution.
Without the interpolation line in U.S. Pat. No. 4,857,929, the number of switches for each binary bit of progressively increased binary significance is double the number of switches for the binary bit of immediately lower binary significance. By providing the interpolation line, the output on the interpolation line from one binary bit can be introduced to an array of reduced complexity for the binary bit of next highest binary significance, thereby reducing the complexity of the switches for such next highest binary bit and the binary bits of even higher binary significance.
Even with the inclusion of the interpolation line and the reduced complexity of the switches for the next highest binary bit and the binary bits of even higher binary significance, the converter of U.S. Pat. No. 4,857,929 does not have as high a frequency range as would sometimes be desired. This has resulted from the fact that the number of switches in the converter is still relatively large and that each of these switches provides distributed capacitances which cumulatively limit the frequency of response of the converter. Another problem associated with the large number of switches is that a large quantity of charge is required to drive the switches from the reference potential source, making that source difficult to stabilize when operating at high frequencies.
Yet another object of this invention is to significantly reduce the number of switches required for an array of equivalent resolution and speed, thereby significantly reducing total array capacitance and reference drive current.